The present invention relates to a constant-current generating circuit and, more particularly to a temperature dependent constant-current generating circuit suitable for a feed forward controlled laser driving circuit for maintaining constant light emission characteristics and/or small time jitter characteristics of an optical output device, such as an optical transmitter or optical links, using a semiconductor light emitting element as a light source, and a driving circuit for driving a semiconductor light emitting element, such as a semiconductor laser diode, using the same.
In recent years, optical telecommunication and optical data links have rapidly been spread. In an optical transmitter used for these purposes, a semiconductor laser (laser diode: LD) directly generates an intensity-modulated light signal. The light signal is transmitted via an optical transmission medium such as an optical fiber.
Particularly, fiber-optic subscriber systems such as FTTH (Fiber To The Home) designed for home optical telecommunication, and transmission modules used for optical data links employ, as an intensity modulation method, a driving method of keeping a high ON/OFF intensity ratio of the light signal (extinction ratio) by supplying a DC biased pulse current to the LD. The DC bias current flowing through the LD should keep slightly lower than the threshold current in the light OFF state, and the pulse amplitude is large enough to oscillate the laser in the light ON state, and to obtain a necessary output intensity.
When a transmission signal speed is relatively low, the systems and the modules may use a zero bias driving method of completely nullifying the bias current in the OFF state of the LD. However, as the frequency of the transmission signal increases, the zero bias driving method becomes difficult to directly apply due to the following reason.
Letting .tau. be the carrier life time of an LD in use, Ith be the threshold current of the LD, Ib be the DC bias current flowing through the LD, and Ip be the pulse current amplitude to get the required transmission signal, the laser oscillation delay time Td of the LD is given by EQU Td=.tau..times.ln(Ip/(Ip+Ib-Ith)). (1)
In general, .tau..multidot. is on the nsec order. For a signal transmission rate of 100 Mb/s or more, a parentheses value of the logarithmic term of equation (1) must be suppressed to 0.1 or less. To realize this in a case of the zero bias state (Ib=0), the ratio Ip/Ith must be 0.1 or less, in other words, the value Ip must be 10 times larger than the value Ith or more. The pulse current amplitude Ip is inevitably set much larger than a value enough to obtain a necessary laser intensity. As a result, the driving circuit must be required a high power, and the power consumption increases.
To the contrary, a pseudo zero bias driving method of always flowing, through the LD, a DC bias current Ib slightly smaller than the threshold current Ith is more advantageous because a high ratio Ip/(Ith-Ib) can be easily attained even if the pulse current amplitude Ip is not so large compared with the zero bias driving method. Therefore, the use of the pseudo zero bias driving method can easily achieve shortening the laser oscillation delay time of the LD, ensuring a high frequency operation, and obtaining a high extinction ratio.
Even in the pseudo zero bias driving method, however, the DC bias current Ib may be difficult to control. Letting T0 be the characteristic temperature of a specified laser threshold current in use, and Is be the threshold current at a temperature T=Ts (standard temperature), the threshold current Ith at an arbitrary temperature T is given by EQU Ith=Is.times.exp((T-Ts)/T0). (2)
The threshold current Ith nonlinearly responses upon a temperature change. For example, in an InP-based LD, the characteristic temperature T0 is several tens to a hundred, and thus the threshold current Ith exhibits a change several to 10 times for a temperature change of 100.degree. C. To make the DC bias current Ib to follow just below the threshold current Ith and keep the difference between these currents almost constant in order to realize pseudo zero bias driving of the LD, the DC bias current generating circuit itself must be the same dependency on temperatures as in the threshold current Ith.
However, no prior art realizes a simple DC bias current generating circuit which can accurately follow temperature variations in threshold current Ith and can be applied to LDs having various characteristic temperatures. For example, a conventional temperature compensation method for the threshold bias current of the LD includes a method of checking the differential value of the DC bias current, and searching and fixing the inflection point near the threshold current, and a method of monitoring the actual light emission intensity of the LD and feeding it back to the DC bias current. These methods require a large scale detection/feedback circuit, so they cannot be applied to purposes in which ICs must be compact, such as LD driving circuits for fiber-optic subscriber systems including FTTH and optical links.
In the LD, not only the threshold current Ith but also the light emission intensity has temperature characteristics. It is known that the light emission intensity can be expressed by an exponential function which decreases together with the temperature using a characteristic temperature T0' as a constant. The characteristic temperature T0' representing the temperature dependency of the light emission intensity of the LD is as high as about several hundred, unlike the characteristic temperature T0 representing the temperature dependency of the threshold current. For this reason, the light emission intensity does not greatly depend upon a temperature change, compared with the threshold current, but often requires temperature degradation compensation. Conventional optical telecommunication has used an APC (Automatic Power Control) circuit for keeping the light emission intensity of the LD constant in order to suppress degradation of the signal quality by the uniform magnitudes of optical transmission signals. A large scale APC circuit of monitoring a part of the LD output with a PD (PhotoDetector) and feeding it back activity to the LD for the purpose of strict control is popularly required.
Since recent improvements of LD performances result in uniform and stable physical characteristics, the characteristic temperature of the LDs is regarded almost constant between elements of a specified product, and a feed forward stabilization circuit is being used. That is, to easily compensate the light emission intensity degradation of the LD by a temperature change, the light emission intensity of the LD is controlled by increasing LD driving current generated by a current source with characteristic temperature. T0'. Thereby the lowering of efficiency of the light emission of the LD is compensated under this passive feed-forward-control.
As the temperature compensation method for the light emission intensity of the LD in the feed forward APC circuit, Jpn. Pat. Appln. KOKAI Publication Nos. 3-214935 and 8-139410, and the like have disclosed that the characteristics of the LD are grasped in advance, and (a) the light emission intensity is roughly approximated using the temperature dependency of the Si diode in the IC; (b) the approximation precision is increased by selecting an appropriate thermistor; (c) the light emission intensity is approximated using a polygonal line by switching several different resistors; or (d) the characteristics of the LD are stored in a memory, and the light emission intensity is strictly adjusted using a D/A converter. Jpn. Pat. Appln. KOKAI Publication No. 9-270507 has disclosed a combination of a voltage source as a modification of a bandgap reference voltage source, an emitter follower, and a current feedback amplifier.
In any of these methods, however, compensation characteristics against a temperature change are inaccurate, and the temperature range is limited. The number of adjustment portions of the circuit is large to adjust characteristics, and adjustment itself is cumbersome. Some of these methods which pose a smaller number of problems require a complicated and large scale control circuit to increase a chip size, or cannot be flexibly applied to LDs having slightly different characteristic temperatures except for specific LDs.
Recently, along with higher performance of multimedia equipments, demands arise for a low power consumption of optical interconnects capable of passing a high speed signal having a frequency of 100 Mb/s or more, instead of a twisted pair cable and a coaxial cable. To meet demands for lower power consumption on the system side, the power supply voltage in use must be low in the driving circuit for driving a light emitting semiconductor element such as a semiconductor laser diode used in an optical interconnect.
In the driving circuit for driving a light emitting semiconductor element, as the power supply voltage decreases, the operation margin of the internal circuit decreases, and the operation margin of the application voltage to the light emitting semiconductor element decreases. Particularly in a differential current switching circuit generally used on the output stage of the driving circuit, the emitter voltage of the transistor constituting the switching circuit rises in a high-temperature operation range in addition to temperature variations in emitter follower circuit on the pre-driver input stage of the switching circuit on the output stage. If the voltage between the collector and emitter necessary for a high frequency operation is kept constant, the application voltage to the load inevitably decreases by an increase in the emitter voltage, resulting in a small operation margin of the light emitting semiconductor element.
Further, in light emitting semiconductor elements such as a semiconductor laser diode and a light emitting diode, as the temperature rises, the light emission efficiency decreases, the current to be injected to the element increases, and the voltage to be applied increases.
Accordingly, the conventional driving circuit cannot meet demands for a decrease in power supply voltage.
As described above, the circuit for generating a DC bias current capable of faithfully following temperature variations in threshold current of the semiconductor laser diode which exponentially changes with temperature is required in the pseudo zero bias driving method of always flowing a DC bias current slightly smaller than the threshold current through a semiconductor laser diode in order to shorten the oscillation delay time, ensure a high frequency operation, and obtain a high extinction ratio so as to realize a high-speed optical data link with a small error rate of transmission. It is, however, difficult to realize this DC bias current generating circuit by the conventional technique. Temperature compensation characteristics can be applied to only a semiconductor laser diode having specific characteristics. The DC bias current generating circuit requires a large scale detection/feedback circuit, resulting in a high cost. At the same time, it is essentially difficult to downsize the DC bias current generating circuit.
In the conventional technique, various methods of temperature-compensating the light emission intensity of a semiconductor laser diode in the feed forward APC circuit have been proposed. In any of these methods, however, compensation characteristics against a temperature change are inaccurate, and the temperature range is limited. The number of adjustment portions is large to adjust characteristics, and adjustment itself is cumbersome. A complicated circuit is required to increase the chip size. In addition, some methods cannot be applied to semiconductor laser diodes having different characteristic temperatures except for semiconductor laser diodes having specific characteristics.
In the conventional driving circuit for driving a light emitting semiconductor element such as a semiconductor laser diode at a very high frequency, as the power supply voltage decreases particularly in a high-temperature operation range, the operation margin of the internal circuit decreases, and the operation margin of the application voltage to the light emitting semiconductor element decreases. When the light emission efficiency decreases, the current to be injected to the element increases, and the voltage to be applied increases. The driving circuit cannot meet demands for a decrease in power supply voltage.